The present invention relates to an image forming apparatus such as a copying machine which can select a desired color copying operation with which to perform color imaging.
In recent years, copying machines have been proposed which can form copy images having different colors by replacing different color developing devices or units including a developing device with other such devices or units. However, in such copying machines, replacement of the developing devices or units is very cumbersome, resulting in inconvenience. Thus, copying machines having the arrangement shown in FIGS. 1 to 6 have been proposed.
Referring to FIG. 1, reference numeral 1 denotes a photosensitive drum. A developing unit 2 is disposed adjacent to the drum 1. The unit 2 comprises developing device receptors 3a to 3d, also shown in FIG. 2. Developing devices 4a to 4d are respectively set in the receptors 3a to 3d. The devices 4a to 4d store toners of different colors. The receptors 3a to 3d are provided on a cross-shaped rotating frame 5, as shown in FIGS. 3 and 4. A stationary gear 6 is coaxially fixed to the frame 5, and the frame 5 is rotatable about the center of the gear 6. Four planet gears 7a, 7b, 7c and 7d are axially and rotatably supported at middle portions of respective stems of the frame 5 while meshing with the stationary gear 6. Four other planet gears 8a, 8b, 8c and 8d are rotatably arranged at the distal ends of the respective stems of the frame 5 while meshing with the corresponding gears 7a, 7b, 7c and 7d. The receptors 3a to 3d are fixed to rotating shafts 9a to 9d of the outer gears 8a to 8d, respectively.
The gears 7a, 8a, 7b, 8b, 7c, 8c, 7d and 8d have the same number of teeth. Since the gear 6 is fixed to the frame 5, even if the frame 5 is rotated the gears 8a to 8d are not rotated about the axes thereof. For this reason, the receptors 3a to 3d (the devices 4a to 4d) are revolved around the gear 6 and kept parallel to each other. Thus, one developing device (e.g., the device 4d) selectively opposes the drum 1. Note that the frame 5 is rotated by a motor 12 through a deceleration means such as gears 10 and 11.
The developing unit 2 is movable along a direction such that it can approach and move away from the drum 1 by means of a moving mechanism (not shown). In the copy mode, as shown in FIG. 1, the selected developing device, e.g., 4b is set to face the drum 1. When selection of one of the devices 4a to 4d is being made, the unit 2 is moved to a position separated by a given distance from the drum 1. When a desired developing device has been selected, the unit 2 is once again moved toward the drum 1.
The devices 4a to 4d have developing device gears 14a to 14d and idle gears 15a to 15d for driving developing rollers 13a to 13d, respectively, while the devices 4a to 4d are themselves driven by a drive gear 16 provided at the side of the housing (not shown). When the unit 2 approaches the drum 1, the drive gear 16 is meshed with, e.g., the idle gear 16b, and a drive force is transmitted to the gear 14b through the gears 16 and 15b.
Guide members 17a to 17d (only members 17b and 17c are shown) are fixed to the receptors 3a to 3d, respectively. A guide pin 18 is fixed to the housing side. The members 17a to 17d can be engaged with the pin 18. Thus, when the unit 2 approaches the drum 1, the selected developing device can be smoothly and precisely guided to obtain a precise positional relationship between the drum 1 and the selected developing device.
However, with the above arrangement, when the devices 4a to 4d are revolved and kept parallel to each other, the gears 6, 7a to 7d and 8a to 8d are used as a selection means for selecting one developing device to oppose the drum 1. For this reason, the devices 4a to 4d cannot be smoothly revolved, thus causing noise, backlash and the like.